1. Field of the Invention
This invention relates to a membership function generator required when a fuzzy theory application is implemented by hardware.
2. Description of the Related Art
In recent years, a large number of applications of the fuzzy theory which can quantify indefinite or uncertain knowledge and information for handling have been implemented in such fields as home electric appliances and plant control. The fuzzy theory is put into practical use by software and hardware. Lately, fuzzy reasoning dedicated hardware appears to meet demands for high speed operation, etc. The dedicated hardware is of digital or analog type. The invention relates to the analog type of fuzzy hardware.
In fuzzy theory, indefinite or uncertain information is described by membership functions, thus means for generating the membership functions is required for the hardware. The analog hardware represents membership function values by, for example, analog voltages, and the specific membership function generator is made up of differential amplification circuits, etc.
FIG. 4 shows an example of a membership function generator made up of differential amplification circuits in the art. The membership function generator generates triangle and trapezoid membership functions and has a feature of generating left and right sides of membership functions separately.
In FIG. 4, D1 and D2 are differential amplification circuits for generating left and right sides of membership functions respectively. The left and right side positions of a membership function are given to inputs VL1 and VL2 respectively from an external system. Left and right side gradients can be adjusted separately according to the values of variable resistors RE1 and RE2. The height of a membership function is determined by the product of current value I.sub.O of constant current source Q1, Q2 and the value of load resistor RL. The left and right sides of the membership function generated at the differential amplification circuits D1 and D2 are combined by a minimum value calculation circuit Q3.
The circuitry shown in FIG. 4 enables triangle and trapezoid membership functions of various forms to be generated as shown in FIGS. 5 and 6 by setting values of the inputs VL1 and VL2, variable resistors RE1 and RE2, and current value I.sub.O (or load resistor RL).
On the other hand, however, the membership function generator requires a complicated membership function setting procedure. A number of parameters should be adjusted: The position and gradient of the left side and those of the right side of a membership function and the membership function height. In fact, most of the actually used membership functions are symmetrical triangle and trapezoid functions as shown in FIGS. 7 and 8. For symmetrical triangle membership functions, only three parameters are required: Center position, oblique side gradient, and height of the membership function. Even for symmetrical trapezoid membership functions, only the upper bottom length of the membership function is added to the three parameters. Thus, from the viewpoint of generating symmetrical membership functions, the conventional method of setting the right and left sides of each membership function separately is more complicated than necessary.